Constraints on the Petrogenesis of the Rockford Granite by Chemical and Radiogenic Isotopic Compositions

Abstract

The Rockford Granite is a Paleozoic igneous body located in Coosa County, Alabama. Regionally, between the Brevard zone and the Kowaliga zone, there lies intense faulting and shearing, which has been studied extensively relative to the tectonic relationship of the Appalachian Mountains. However, the igneous rock body that cuts across those structures has not been researched fully and its origins are not well understood. The aims of this study are (1) to characterize the Rockford Granite on the border between granite and granodiorite and (2) to further constrain the genesis and petrogenic conditions of the igneous rock body. This thesis reports petrographic, major element, trace, and rare earth element, 87Sr/86Sr and 143Nd/144Nd compositions and biotite chemistry for the Rockford granite. Data collection of rare-earth elements on all samples has chondrite-normalized LREE-enriched patterns and flat HREE distributions. Samples 21RKF1 and 21RKF2 suggest no fractional crystallization, whereas samples 21RKF3 and 20ROCK1 display minor negative Eu anomalies, indicating moderate fractional crystallization of plagioclase. Our new radiogenic data yield initial 87Sr/86Sr ratios ranging between 0.704537-0.705938 and initial 143Nd/144Nd ratios ranging between 0.512042-0.512215. Their Nd vs Sr plot indicates I-type granite character and suggests that the mantle contributed not only heat but also juvenile mantle materials for the genesis of the Rockford granite. Additionally, using the AU-EMPA, biotite elemental composition data are used to determine the temperature (618-652 °C) and redox conditions (QFM+1.0) for the Rockford granite.